Amplifier system



Feb. 13, 1940. H A, WHEELER AMPLIFIER SYSTEM Filled July 29, 1937 FIG.|.

F\G.4. y T /if/ f f/V/ f V f /fe 30 A Frequency |69 INVENTOR R LD A. WHEELER,

ATTORNEY m Y B so@ anim ox uw 2. 7 6 m.. l E l n w g E .F a 56 6 4 Q WWA M .a nu .m O f.. J w l a ma mm man C nom c a uw. E I. umm. www www? mm www mum mw ww www f n@ D i F. ioni W M; w 4 E8 35am flu L Patented Feb. 13, 1940 UNITED s'rA'rEs. 1 '.1\Tr.1\rr OFFICE y y @Mesem I iraniana? Application Julyzs, '1931, `seran No. 156.296

' 11 claims. (on 17e-#171) lThis invention relates to an arrangement for stabilizing an amplifier system which operates over a range of frequencies but tends to become unstable when subjected to direct current or lvery low-frequency alternating-current pulses or One embodiment of the invention is particu-` larly adapted to systems operating over a veryA wide range of frequencies, the lower frequency limit of which is a very `low.frequency, such as a video-frequency amplifier used in television systems, while ,anotherr embodiment of the invention is particularly adaptable for eliminating the beat note or flutter developed `and. reproduced byy w a modulated-carrier signal receiver tuned to a desired signal when there is present on the same signal channel-an undesired signal of nearlybut not exactly, the same carrier frequency. Asrused in this specification, a low-frequency amplifier K 20 refers to an amplifier for amplifying a range of low frequencies, but such amplifier may also amplify a very broad range of frequencies including high frequencies.

In amplifiers required to operate in a very low-A 2" frequency band, it is sometimes impractical to l use capacitive coupling between successive stages of amplification because very large coupling condensers are required. Furthermore, if it is n desired to amplify a veryfwiderange offrequen- "0 cies, such large condensers, having an appreciable capacitance to ground, place a limit upcn the system, a' change in ambient temperature,`

4" faulty contacts yin the system, fluctuations in cathode temperature, etc. It has previously been necessary to reduce to a minimumV these factors tending to produce an unstable condition in `order to` procure satisfactory operation of an amplier of the type mentioned. l

Furthermore, in the reception of a desired modulated-carrier signal Ycn a givenchannel, it frequently happens that signals of a lower order of magnitude are present on the same channel,

'3" due a weak or distant station operating on Such effect may be caused l the channel. 'I'hese signals may be lmodulated by thesame program so that the modulation frequencies do not cause objectionable interference, butif the carrier of the undesired signal is not synchronized with that ofthe desiredsig- 6 na1,which is usually the case. abeat note or ,'ffiut. ter" of very low frequency will be present in the audio-frequency channelof the receiver due to the heterodyn'e action between the desired and undesired signal carriers. The frequency` dii'fer-` 10 ence between such stations operating on the same "channel and e thus .the beat-note" frequency is usually between zero andfifty cycles. t

It haspreviously been proposed to utilize highpass lters inthe audio-frequency channel of the signal receiver to eliminate the undesirable fiutster described above. A filter circuit producing an attenuation of decibels at 30 cycles with# out appreciable attenuation above 1GO cycles has usually been foundsuiiicient to eliminate most 20 of the utter. In general; however, thefuse `of such filters is impracticable for the reason Vthat high-inductance choke coils or high-capacitance condensers, or both, are required in the It is an object" of the invention, therefore, to

provide-an amplifier capable of operating over a very Wide range of frequencies, the lowerV frequency limit of which is a very low frequency and which is stable when the system is subjected to pulses or surges of lower frequency or direct current.

It is a further object ofthe invention to provide a modulated-carrier signal receiver including a simple arrangement` for-the elimination of the beat note or "flutter produced therein between a desired signal and an undesired` signal e which is present on the same channel but which is not synchronized. with the desired signa...

In accordance with the invention an amplifier includes means for feeding back a voltage from l one point in the amplifying system to a preceding pointy of the system, the feedback being degenerative primarily at frequencies below the lower limit of thedesired range, that is, frequencies of the disturbances causing the amplifier to be unstable or producing an undesirable beat note or fiutter. The feedback becomes decreasn ingly degenerative with increasing frequency bel5()` stability or produce@ 0bjectionable beatnote 55f auaticv dit appended claims. f

Fig. 1 of the drawing is a circuit diagram illustrating an embodiment of the invention as applied to a resistance-coupled direct current arnpliiier; Fig. `2 isa diagram illustrating certain characteristics of the system of Fig. 1; Fig. 3

is a circuit diagram, partly schematic, illustrati ing anembodiment of the invention as incorporated in a modulated-carrier signal receiver of the superheterodyne type; and Fig. 4 is a diagram illustrating certain characteristics of the receiver of Fig. 3.

Referring now to Fig. 1 of the drawing, there is shown a circuit diagram of a resistancecoupled direct current ampliiier having three stages of amplification comprising vacuum tubes in, II, and I2. The anode of tube I is connected through load resistance I4 and decoupling resistance I 5 to a ysource of supply voltage, such as a battery I6; a decoupling condenser I1 is connected between the common terminal oi' resistors i4 and'IS and ground. The anode of tube IU is connected to the control grid of tube I I through a circuit comprising a resistor I8 and a condenser I9 in parallel, while the control grid of tube II is connected to its cathode through a grid-leakv resistor 20 and a bias source 2|.

The output circuit oi tube II is ccnnected'to the input electrodes of tube I2 in a. manner similar to that described for coupling tube |0- to tube II. Thus the anode of tube II is connected through load resistance 22 and decoupling resistance 23 to the source I 6: decoupling condenser 24 is connected between the common terminal of resistors 22 and 23 and ground; the anode of tube II is connected to the control grid of tube I2 through a circuit comprising a resistor 25 and a i condenser 26 in parallel, while the control grid of tube I2 is connected to its cathode through gridleak resistor 21 and a bias source 28. A coupling system such as has been described above is clescribed in British Patent No. 456,450 to White.

Briefly, the amplifier described above comprises a plurality of amplifying tubes in which the external load impedance of each tube, together with the anode-cathode impedance of the tube, can be represented over a range of frequencies down to and including zero frequency by a pure resistance R1 (resistors I4, 22) in series with an impedance Z1 (resistor I5 and condenser I1, resistor 23 and condenser 24). The anode of each tube is connected to the control grid of the succeeding tube through a coupling impedance Z2 (resistor I8 and condenser I9, resistor 25 and condenser 26),.

In order to secure a uniform amplication over the range of frequencies to be amplified, the circuit in such amplifier is preferably arranged such that:

' z fs Z1 R1 when Ra represents grid resistors 20 or 21.

Corning now tothe parts of the system involving the present invention, the' system is stabilized by feeding back a voltage from one point in the system to a preceding point in the system. More particularly, the feed-back path is from the comcircuits.

d' mon terminal of resistors 22 and 23 to the common point of resistors Iii and I5 and includes resistors 29, 30, and 8|; shunt condensers 2?' and 30' connected between the common terminals of resistors 29, 30, and 3i, respectively, and ground.

forming a conventional ladder lter.

(Ris't-RORia-Rzo where Rar-:Rza and the subscripts identify the circuit elements of Fig. 1. The first equation is the condition for equality of the time constants ofthe correcting or compensating condenser These time constants should have a value of the same order of magnitude as the period of a frequency considerably greater-than the lowest frequency to b'e amplified andco'siderably less than the highest frequency amplified. This avoids interference between the condenser correcting circuits and the feed-back circuit which operates at a very low frequency.

It alsoy avoids interference between the condenser correcting circuits and the inherent capacity of the tube which becomes important at very high frequencies.

In considering the operation of the feedback circuit, it will be seen that the output voltage of tube I0 is subjected to a reversal of polarity as it is transmitted through the'tube II so that the output voltage of tubel II is substantially in phase opposition to the input voltage of tube II. The feed-back circuit applies to the input oi' tube II a voltage derived from the output of tube IIl and having a phase displacement with respect to input voltages of corresponding frequencies which varies as a function of frequency. Thus, for very low frequencies, the impedance of shunt condensers 29' and 30' are very large with respect to the impedances of resistors 29 and @il and the phase of the feed-back voltage is retarded only slightly by the iilter sections of the feed-back circuit; that is, the feed-back voltage is applied to the input circuit of tube II substantially in phase opposif tion with the signal input voltage to this tube so that, at these low frequencies, the system is degenerative. The constants of the feed-back path are such that, at slightly higher frequencies, the impedances of condensers 29 and 30' are substantially less than, and so related to, the impedances of resistors 28 and 30 that the filter network retards the phase of the signal voltage by an amount approaching 180 degrees as a limit and the system becomes progressively less degenerative and actually regenerative; in addition, this feed-back circuit has a time constant proportioned to attenuate or filter out desired signal components of the frequency range. It will also be seen that there is a direct current feed-back path through series-connected resistors 29. 38,

casacca and 3i, these resistors being proportioned materially to reduce the gain of the amplier at zero frequency.

In Fig. 2 there are shown three curves representing the frequency-response characteristics of the system of Fig. l. Curve a indicates a uniform responsef'over the operating frequency range and i o V embodying the present invention, the undesirable` represents the normal operation of the ampiier Without the feed-back circuit; curve b represents the operation of the amplifier with the feed-back circuit of Fig. 1; while curve c is included as an example of the effect which may be obtained by using a single ladder section comprising a series resistor and a shunt capacitor in the feed-back circuit, as, for example, omitting the elements 38, 38 of Fig. 1. An increase in the number of .ladder sections in the feed-back circuit may cause the frequency-response characteristic of the system to be slightly irregular above the 'cutoff frequency and tends to produce a sharper cut oil characteristic.

Referring now to Fig. 3, there is shown a circuit diagram of a complete modulated-carrier signal receiver of the superheterodyne type embodying the invention, in which certain conventional portions are schematically shown, since,- per se, they form no part of the invention. The receiver comprises in cascade an antenna-ground circuit 32, a radio-frequency selector and ampliiler 33, a frequency changer or oscillator-modulator 34, an intermediate-frequency amplifier 35,

` a detector and automatic amplification control rectifier 36, an audio-frequency amplifier of three stages indicated generally at 31, nand aloudspeaker 38. An automatic ampliilcation control bias derived from unit 36 is applied by way of connection 36 to the grids of one or more oi the tubes of the several stagesof the receiver to maintain the input to the detector 36 within a relatively narrow range for a wide range of received signal intensities.

Audio-frequency amplifier 314comprises an ampliiier tube 39, havingits input coupled to unit 36 through a voltage divider volume control 4D and its output circuit coupled through load resistor 4| and coupling condensers 42 and 43 in parallel to an amplier tube 44,` theoutput circuit of which is coupled through transformer 45, 46 to a stage of push-pull amplification comprising tubes 4l and 4B. The common output circuit of tubes circuit, Fig. 3. it will be` seen that the phase ci' the signal is subjected to two reversals as it'is transmitted through tubes 39 and 44 and to `an tion, the system described above includes the features of a conventional superheterodyne receiver. The operation of such a receiver being well understood in the art, a detailed explanation thereof is deemed to be unnecessary. Briefly, however, a desired modulated-carrier signal intercepted by antenna 32 is amplified in radio-frequency amplifier. 33 and converted in oscillator-modulator 34 to an intermediate-frequency modulated-carrier signal. `This signal is ampliled by intermediatefrequency amplifier 35 and translated therefrom `to detector 36, where the audio frequencies of modulation and automatic amplification control potentials'are derived. The audio frequencies of modulation' are successively amplified by ampliiler stages 39 and 44 andthe push-pull stage of amplification comprising tubes 41 and 48 and are reproduced by loud-speaker 38 in a conventional manner. The automatic amplication control applied to one or more of the tubes in the precedmaintain the signal outputhof ,the receiver within a'relativvelynarrow range for awide range of i received signal amplithdes, in a conventional manner;y

Coming now .to the parts of the system of Fig. 3

beatl note or flutter described above is substantially eliminated by feeding back a voltage from ing stagesof the receiven-l-from unit 36 serves to 5 one pointfin the audio-frequency channel 31, to a f preceding point of the channel. More particulai-ly, the feed-back path is from the secondary Winding of transformer 45, 46 to the input circuit 'of amplier 39 and includes series-connected switches 5| and 52, resistors 53, 54, 55, and 56 and condenser 51 with shunt condensers 56, 58, and 60 connected between the successive junc- ``tions' of resistors 53,` 54, 55, and 56 and ground to form the` conventional ladder-,type filter. Ir1 order 'thatn condenser 42 may be effectively removed from the coupling circuit between tubes 39 and 44 when the feed-back circuit of the in# i vention is in operation, there is provided a switch 6i ganged ior unicontrol with switches 5I and 52,

as indicated at U in thedrawing. A resistor 62,..

suilciently large to prevent condenser 42 `from "In considering the operation of the feed-back additional reversal in transformer 46 so that the phase of the output voltage of transformer 45, 46,

is substantiallyiin phase opposition to the input Y A, Megohms Resistor 41 0.1l Resistor 53;n 0.5 Resistor 54......A f 1.0 .Resistor 55 0.5 Resistor 56 2.0 Resistor 62 -..-h; -lf 5.0

. i Microfarads l Condenser 42` .05 Condenser 43 -s .007 Condenser `5'7 .05 Condenser 58 .005 Condenser 59 .01 Condenser 60 .01'

voltage of tube 39. The feed-back circuit applies f i to the input of tube 39 a voltage derived from the output of transformer 45, 46 and having a phase d'splacernent with respect to input voltages of corresponding frequencies` whichvaries as a func,- tion `of frequency. Thus, for very low audio fre'- qucncies the impdances of shunt condensers 58, 59,`and 60 are'very largeA with respect to those -of series resistors 53, 54, and 55 and the phase of the voltage fed back is retarded only slightly by each of the three ladder sections of the network 5I-G3 and is applied to the input circuit of tube as substant'auy in phase opposinan with the signal input voltage, so that at these low` frequencies the system is degenerative. The circuit i constants of the feed-back path are such that in the vicinity of cycles the'impedances of condensers 58, 59, and 68 are substantially less than,

and so related to, the impedances or. the resistors as," s4, and ss that each or the three ladder' sections of the network retards the phase of the signal voltage approximately 60 degrees so that the nal feed-back voltage is in phase with the input voltage and the system becomes regenerative.

In order that the regenerative effect of the feed-back circuit shall have no appreciable effect in increasing the over-all gain of audio-frequency amplier 31 at frequencies above 100 cycles, condensers 51-60, inclusive, are so proportioned that their reactances are suiiiciently low at such frequencies that practically n o voltage is coupled through the feed-back path to the input circuit of amplifier 39. Furthermore, the shunt eect of these condensers on the input circuit of the pushpull amplifier comprising tubes 41 and 48 reduces the output of the receiver by a slight amount at the higher frequencies which tends to compensate for the regenerative effect of the feed-back circuit and is not objectionable. At the saine time,

the regenerative action of the feed-back path in the vicinity of 100 cycles tends to produce a marked peak in the response characteristic at this frequency. This is avoided by the opening of switch 6i upon closing switches 5| and 52 to place the feed-back path in operation. Opening of switch 6I is effective to remove coupling condenser 42 from the circuit and, therefore, to reduce the capacitive coupling between tubes 39 and 44, thus reducing the response of the system at the lower audio frequencies.

In Fig. 4 there are vshown the frequencyresponse characteristics of the receiver of Fig. 3; curve d without the feed-back circuit described above, curve e with the feed-back circuit, and curve f with the feed-back circuit and coupling condenser 42 both removed It is apparent from Fig. 4 that a substantial attenuation is introduced into the receiver at 30 cycles by the feed-back circuit of the invention. It is also apparent that at frequencies above 100 cycles, the receiver characteristic is not materially altered. It has been found that a receiver characteristic as illustrated at e is effective substantially to eliminate the undesirable beat note or flutter described above. It Will be understood that the feed-back circuit of this invention can be connected across one or more stages of the amplifier toY which it is applied and that more than-one feed-back path can be employed if desired.

While there have been described what are at present considered the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from this invention, and, therefore, it is aimed inthe appended claims to cover all such changes and modifications as fall within the true spirit and scope of this invention.

Whaty is claimed is:

l. A signal amplifier for amplifying a desired range of signal frequencies including low-frequency signals and comprising input and output circuits, a degenerative very low-frequency feedback circuit between said output and said input circuits, said feed-back circuit including a plurality of series resistors and shunt condensers and having a time constant proportioned to filter out desired signal components of said frequency range, whereby the desired range of. frequencies is amplified without substantial interference from undesired very low-frequency fluctuations in the system.

2. A signal-amplifying system for amplifying a desired range of signal frequencies including lowfrequency Signals, a feed-back circuit 'from one point of said system to a preceding point thereof, a resistance-reactan'ce network in said feed-back circuit having a time constant proportioned to shift the phase of the feed-back voltage so that said`feedback is degenerative at very low frequencies and is regenerative at higher frequencies, whereby undesired low-frequency fluctuations in said system are substantially eliminated, said feed-back circuit having an attenuation proportioned to reduce said feed-back voltage to a negligible value over. said desired signal-frequency range.

, 3. A signal-amplifying system for amplifying a desired rangeof signal frequencies comprising a direct current amplifier and a degenerative direct current feed-back connection between one point of said system and a preceding point thereof, said connection including series resistors and at least one condenser connected between a junction of said resistors and the return of said feedback circuit, said resistors and condensers having a time constant proportioned to lter out desired signal components of said frequency range,`whereby the desired signal components are amplified without interference from undesired low-frequency and direct current uctuations in said system. l

4. A signal amplifier for amplifying a wide range of frequencies comprising a vacuum tube having input and output terminals, a conductive input circuit coupled to said input terminals, a conductive output circuit coupled to said output terminals, said input and output circuits introducing a minimum capacitance between said input and between said output terminals, and a feed-back circuit from said output circuit to said input circuit comprising a direct current connection for degenerative feedback of direct current fluctuations and a low-pass filter having a time constant such that its cutoff frequency is below said range and signal components of said range are filtered out, whereby alternating currents of said range are amplified without interference from direct current or very low-frequency iuctuations in the system.

5. In a modulated-carrier signal receiver for receiving a desired signal but subject to the reception of an undesired signal on the same signal channel which is not synchronized with said desired signal, an amplifying system comprising an audio-frequency channel, an amplifier in said audio-frequency channel having input and output circuits, a feed-back circuit from the output circuit to the input circuit of said amplifier, the reactive constants of said feed-back circuit having a time constant so proportioned that said feed-back circuit is degenerative at low audio frequencies and regenerative at higher audio frequencies and has an attenuation proportioned to reduce said feed-back to a negligible value at said higher frequencies, whereby the beat note between said desired and undesired signals is substantially eliminated and the receiver character-A istics at said higher frequencies are not substantially altered by said feed-back circuit.

6. In a modulated-carrier signal receiver for receiving a desired signal but subject to the reception of an undesired signal on the saine signal channel which is not synchronized with said desired signal, yan amplifying system comprising an audio-frequency channel, a feed-back circuit from one point of said audio-frequency channel to a preceding point thereof, and a resistancereactance network in said feed-back circuit havis n l sired signal components and to shift the phase of the feed-back voltage so that said voltage is degenerative primarily at low audio frequencies,

thereby substantially to eliminate the beat note vbetween said desired and undesired signals.

7. In a modulated-carrier signal receiver for` lreceiving a desired signal but subject to the rel ception of an undesired signal on the same signal channel which is not synchronized with said desired signal, an amplifying system comprising an audio-frequency channel, a feed-back circuit from one point of said audio-frequency channel to a preceding point thereof, and a resistancecapacitance network in said feed-back circuit having a ltime constant proportioned to ilter out desired signal components and to shift the phase of the feed-back voltage so that said voltage is degenerative primarily at low audio frequencies, thereby substantially to eliminate the beat notes between said desired and undesired signals.

8. In a modulated-carrier signal receiver for receiving a desired signal but subject to the reception of an undesired signal on the same signal channel which is not synchronized with said desired signal, an amplifying system comprising an audio-frequency channel, a feed-,back `circuit from one point of said audio-'frequency channel to a preceding point thereof, and a resistancecapacitance network of the ladder type comprising series resistors and shunt condensers having 'a time constant proportioned to filter out vdesired signal components and to shift the phase of the feed-back voltage so that said voltage is degenerative primarily at low audio frequencies. thereby substantially to eliminate the beat notes between said desired and undesired signals.

9. In a modulated-carrier signal receiver for v receiving a desired signa1 but subject to the reception of an undesired signal on the same signal channel which is not synchronized with said desired signal, an amplifying, system comprising an audio-frequency channel, aA feed-back circuit from one point of said audio-frequency channel to a preceding point thereof, means in said feedback circuit having a time constant proportioned to vfilter. out desired signal components and to shift the phase of the feed-back voltage so that saidvoltage is degenerative at low audio `frequencies andis regenerative at higher audio frequeni cies, whereby the beat vnote between said desired and undesired signals is substantially eliminated,

and means eective primarily at said higher frequencies for reducing the forward gain in said l audio-frequency channel, thereby` partially to compensate for the regenerative effect of said feed-back circuit.

10. In a modulated-carrier signal receiver for receiving a desired signal but subject to the reception of an undesired signal on the same signal channel, an amplifying system comprising an audio-frequency channel, a forward coupling capacitance means in said audio-frequency channel, means for selectively connecting a feed-back circuit from one point of said audio-frequency channel to a preceding `point thereof, means in said feed-back circuit for shifting the phase of the feed-back voltage so that said voltage is degenerative at low audio frequencies and is regenerative at higher audio frequencies. whereby the beat note between said signals is substantially eliminated, and means for reducing thevalue of said coupling capacitance means when said feedback circuit is included in said audio-frequency channel, thereby partially to compensate for said regenerative feed-back.

11. In a modulated-carrier signal receiver for receiving a desired signal but subject to the reception of anundesired signal on the same signal channel. an amplifying system comprising an audio-frequency channel, a forward coupling ca-` pacitance lmeans in said audio-frequencychan nel, switchingmeans for connecting a feed-back` circuit from one point of said audio-frequency channel to a preceding point thereof. means in said feed-back circuit for shifting the phase of the feed-back voltage so that said voltage is degenerative at low audiofrequencies and is regen' 

